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Handbook of Turbomachinery Second Edition Revised - Ventech!

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increase <strong>of</strong> required suction performance from both pumps. This resulted in<br />

the introduction <strong>of</strong> a device known as an inducer at the inlet <strong>of</strong> both fuel and<br />

oxidizer pumps. An inducer is simply a single-stage axial pumping element<br />

whose diameter matches that <strong>of</strong> the main impeller inlet. For those who have<br />

never seen one, the geometry <strong>of</strong> an inducer is somewhat reminiscent <strong>of</strong> a<br />

screw or propeller. The inducer is usually fairly lightly loaded and imparts<br />

just enough head to the pumped fluid that the main impeller inlet does not<br />

experience adverse effects due to cavitation. Although the inducers fitted to<br />

the Thor engine pumps were somewhat primitive by today’s standards, they<br />

caused the suction performance capability <strong>of</strong> these pumps to be nearly<br />

tripled.<br />

Lastly, we must address the problem <strong>of</strong> coupling turbine and pump<br />

shafts. In the design <strong>of</strong> the Redstone engine turbopump, turbine and<br />

pumps were on a common driveshaft. This was impossible in the Thor<br />

engine turbopump. In order to increase turbine efficiency and to<br />

minimize turbine weight, the turbine shaft speed was optimized separately<br />

from the pumps. At more than 30,000 rpm, the turbine was running at a<br />

speed that was far beyond the capability <strong>of</strong> centrifugal pump stages to<br />

support.<br />

The only practical answer was to link the turbine and pump shafts<br />

together via a gear reduction system. As it turned out, this gearbox formed<br />

the structural backbone <strong>of</strong> the Thor turbopump. Fabricated from an<br />

aluminum sandcasting, the gearbox contained a 4.8:1 reduction gear train to<br />

link turbine and pump shafts. In addition, it contained provisions for an<br />

accessory drive pad (which could be used by a missile hydraulic pump, for<br />

example), as well as connect points for both centrifugal pumps as well as the<br />

turbine assembly. The fuel pump was bolted to the gearbox, while the LOX<br />

pump was attached by radial pins around the circumference <strong>of</strong> the pump<br />

rear. This technique provided radial load relief for the LOX pump housing<br />

to permit thermal contraction <strong>of</strong> the housing without causing any undue<br />

radial direction loads. It is also worth mentioning that this gear reduction<br />

train proved to be one <strong>of</strong> the largest technical challenges <strong>of</strong> the turbopump<br />

development. The desire to keep inert weight to a minimum caused the<br />

gearbox to be sized as small as was practical. This reduced the gear<br />

diameters, and therefore their pitchline velocity. The transmission <strong>of</strong> in<br />

excess <strong>of</strong> 3,000 hp to a main driveshaft moving relatively slowly through<br />

gears that have low pitchline velocity caused very high forces between gear<br />

teeth. This condition resulted in a number <strong>of</strong> cases <strong>of</strong> premature fatigue<br />

failure <strong>of</strong> turbopump gears. Modifications to the gear tooth involute pr<strong>of</strong>ile<br />

yielded a wider tooth root with greater load capacity, as well as a reduction<br />

in shear loading at the tooth root. This, along with careful processing <strong>of</strong><br />

the gear material and somewhat unique fabrication techniques, resulted in<br />

Copyright © 2003 Marcel Dekker, Inc.

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